This is a pretty terrible reason, but I'm going to throw it out there:
I like real names because they're much more aesthetically pleasing. In
my younger days, I once decided, "Hey, I should get a pseudonym" and I
picked something fairly ridiculous, just because "everyone else was doing
it". I would have appreciated someone to have conked me on the head earlier
and said, "No, that pseudonym is stupid" and made me use something else.
That said, I think I'd be perfectly happy with a policy that preferred
real names, but was willing to take an (obvious) pseudonym if the author
insisted.
Cheers,
Edward